Directional couplers are used for detecting signal properties of propagating high-frequency signals, that is, electromagnetic waves in high-frequency lines. A portion of the high-frequency signals is directionally coupled out of a high-frequency line via a coupling line. These signals can be tapped via measurement connections of the coupling line and evaluated in a detector. This makes it possible to assess the quality of the propagating high-frequency signals. For example, it is possible to assess the magnitude, the transmitted power, or the phase of a high-frequency signal on a high-frequency line. Typical coupling losses, that is, components of the power level of the propagating high-frequency signals that are coupled out can, for example, fall between −3 dB and −20 dB.
Directional couplers are widely used for the directional detection of power differences between an outgoing and returning electromagnetic wave in a signal path, in order to be able to calculate the so-called voltage standing wave ratio (VSWR). The voltage standing wave ratio is a measure of power losses of electromagnetic waves that are caused, for example, by reflection due to mismatched line segments in the signal path having impedance discontinuities.
In electronic communication terminals such as mobile telephones or other wireless transmitting/receiving units, a plurality of frequency ranges (frequency bands) is used to transmit information and data, for example, bands between 700 MHz and 1000 MHz or between 1400 MHz and 6000 MHz.
In conventional solutions, a directional coupler is employed for each frequency band that is used in order to detect signal properties of the propagating high-frequency signals in this frequency band. For example, the power of a propagating electromagnetic wave between a transmitter and an antenna is detected in order to enable a controlled ramping up of the power of a power amplifier in the transmitter. Furthermore, the power of an outgoing and returning electromagnetic wave between the transmitter and the antenna can be detected in order to detect a mismatch and to effect an impedance match between the transmitter and the antenna.
The line lengths of the coupling line and the lines between the directional coupler and a detector that is connected to it for measuring the coupled-out signals generally depend on the respective frequency band and are therefore different. Different line lengths result from the fact that for certain functions, a line length that is dependent on the wavelength, for example, a quarter or half of the wavelength, is specified.
A stable and defined coupling in the directional coupler must be ensured despite different line lengths for different frequency bands. This is true for the detection of both outgoing and returning waves. In order to match a directional coupler to the employed frequency band and if necessary also to the resulting line length between the directional coupler and the detector, a conventional directional coupler can have resistive attenuators between the coupling line and the measurement connections in the direction of the detector. The attenuators are constructed via discrete resistors in a PI or T form.
All of these requirements—different frequency bands, the use of directional couplers in different settings, discrete resistive matching elements—result in a costly directional coupler for different frequency bands that requires a large amount of space.